In cardio-pulmonary resuscitation (CPR) repeated compressions are administered by hand or by apparatus to the chest of the person being resuscitated to maintain circulation and oxygenation of blood. Concomitant with the compressions electrical shocks are provided to the patient to make the heart beat again. Gas-driven reciprocating CPR apparatus have been known in the art and used in practice for a long time; see, for instance, U.S. Pat. No. 3,209,747 (Guentner) and U.S. Pat. No. 3,277,887 (Thomas). Providing compressions of correct depth is an important factor for success of the method.
In the following “Compression Depth” signifies the maximum sternal deflection during a compression/decompression cycle. An appropriate Compression Depth for adult persons corresponds to a sternal deflection of 20%; the compression depth for a chest with an anterior-posterior diameter of 25 cm thus is 5 cm. In contrast “compression depth” in the following refers to a sternal deflection during a compression/decompression cycle smaller than the maximum deflection or to sternal deflection in general.
Shallow compressions may be insufficient to restore circulation and oxygenation while compressions that are too deep may damage the ribs and the soft tissues of the chest. There is thus an optimal Compression Depth or a narrow range of optimal Compression Depths. Administration of compressions of optimal Compression Depth may be controlled by administering compressions of a given force. Alternatively, a desired Compression Depth may be set by an operator; it may be optionally changed during resuscitation. Alternatively, the Compression Depth in a CPR apparatus can be set by limiting the stroke of the piston in the apparatus to the average optimal Compression Depth for an adult person. A given compression force results in a compression to a Compression Depth at which the compression force is balanced by the resistive force of the chest tissues. Since even adult persons differ in their chest anatomy a given compression force may result in compression of varying depth in a group of persons. Therefore the direct determination of Compression Depth during cardiopulmonary resuscitation and its use for control of the apparatus by which the compressions are administered is desirable.
The determination of compression depths during cardiopulmonary resuscitation is known in the art. A corresponding device and a method based on accelerometer measurements disclosed in U.S. Pat. No. 7,118,542. An accelerometer-based compression monitor is placed on the patient's sternum, the arm of a rescuer administrating manual heart compressions or on a compression-administrating part of an automatic CPR device. The chest is then compressed. The accelerometer signal is integrated and fed to a processor, which calculates the compression depth from the signal by use of complex algorithms. The accelerometer is electrically connected to the processor.
In the administration of repeated compressions in cardiopulmonary resuscitation the use of apparatus based on a reciprocating piston provided with a chest compression pad and mounted in a cylinder is known. The piston is driven by a compressed gas. An apparatus of this kind dedicated for use by off-hospital medical emergency teams is disclosed in US 2003/0181834. The Compression Depth administered with such an apparatus is limited by physical means comprised by the apparatus and set from start to from about 40 mm to about 50 mm for an adult person.
A problem with CPR apparatus driven by a compressed gas is limited gas supply, since the apparatus may be used for an hour or more to provide compressions to the patient during transport to a hospital by, for instance, an ambulance.